Social conflict is a major source of stress, and perhaps disease, in a variety of species, including humans. The outcome (i.e., whether an individual is a "winner" or a "loser") of even a relatively brief exposure to social conflict can produce profound and long-lasting changes in subsequent behavior and physiology. For example, when a Syrian hamster is defeated during a brief encounter in the home cage of a larger, aggressive opponent, the defeated hamster will subsequently fail to display normal territorial aggression in its own home cage even when the intruder is a docile, younger animal. This dramatic, yet easily obtainable, change in social behavior has been called conditioned defeat, and we propose that it represents an ethologically relevant and valuable addition to current animal models wherein behavioral responses related to stress, fear and anxiety are studied. The long term goal of this project is to define the neural events that trigger the profound changes in behavior that occur during conditioned defeat. There are currently no published data on the neural elements that regulate conditioned defeat. We propose that the basolateral amygdala, central nucleus of the amygdala and the bed nucleus of the stria terminalis (BNST) are critical for the acquisition and expression of conditioned defeat. This hypothesis was developed from several converging lines of evidence from studies on the neurobiology of defensive/submissive behaviors and of fear, as well as from strong preliminary data generated in our laboratory. The proposed studies will not only provide the first information on the anatomical sites mediating conditioned defeat, but they will also provide initial information on the neurochemical signals involved.